Remove useless alignment from MpegEncContext.intra_scantable

The first member of struct ScanTable is a simple pointer, extra alignment
of which serves no purpose.  The alignment specifier was added along with
some Altivec optimisations also adding a 16-byte-aligned array at the end
of struct ScanTable.  Presumably the redundant, outer alignment was added
by mistake.  The inner one is clearly sufficient.

Originally committed as revision 19724 to svn://svn.ffmpeg.org/ffmpeg/trunk